1. Field of the Invention
The present invention relates to wideband microwave polarization couplers and, more particularly, to polarization couplers which provide coupling for dual linear polarized signals having frequencies which extend over extremely wide frequency bands.
2. Description of the Prior Art
It is well known that the information capacity of a given guided wave microwave transmission system can be increased by utilizing the two dominant cross polarized modes of the main guide of the system to propagate two different waves at a common frequency. In any such dual mode system it is often necessary to selectively couple the mode of one polarization to and from the main guide to the exclusion of the other mode of polarization. Polarization couplers for performing this selective coupling function are known in the art. Typically, such couplers comprise a main guide, a side-arm guide joined to the main guide, and a discontinuity disposed within the main guide such that the excluded mode is transmitted through the main guide and the coupled mode is diverted into the side-arm guide. Generally speaking, these prior art couplers have performed satisfactorily only over single frequency bands such as, for example, the 4 GHz common carrier band for the coupled mode. None of these couplers, therefore, are capable of "ultrawideband" operation for the coupled mode. The term "ultrawideband" is used herein to mean a frequency band for which the ratio of the highest to the lowest frequency contained therein is at least 1.74. Thus, e.g., a band extending through the 4 GHz and the 6 GHz common carrier bands would be such an ultrawideband.
A polarization coupler is disclosed in U.S. Pat. No. 2,961,618 granted on Nov. 22, 1960 to the applicant hereof and assigned to the assignee hereof wherein the bandwidth of the excluded mode of polarization can be extended over an ultrawideband of frequencies (i.e., from 3.7 GHz to 11.7 GHz).
For other patents which disclose arrangements for increasing the bandwidth of the coupled mode of polarization coupler see, for example, U.S. Pat. Nos. 2,975,380 issued to H. Scharfman on Mar. 14, 1961, and 3,327,250 issued to G. B. Sleeper, Jr. on June 20, 1967. In the Sleeper, Jr. patent such a result is realized by disposing two septa, which are offset from each other, at the junction of the main guide and side-arm guide of the coupler. In the Scharfman patent, on the other hand, a plurality of posts or rods are positioned at the junction to obtain the increased bandwidth. The arrangements of the latter two patents, however, still provide a coupled mode which is limited in bandwidth but provides improvement over the prior art polarization couplers.
The problem remaining in the prior art is to provide a polarization coupler for which the coupled mode of polarization can have an ultrawide frequency band.